

package main

import "hash"


/*
16-bit CRC , CRC-CCITT, polynomial 0x1021.
初始值固定是 0.
结果是 32 位整数, 低 16 是 crc 结果,  高 16 bits 是固定 0xCAFE .
*/



type ccitt struct  {
	ctbl []uint16
	crcv uint16 
	offs uint32
}


func ( ctt * ccitt )Write( blk []byte) (int, error)  {
	
	for _, val := range(blk) {
		idx := (ctt.crcv >> 8) ^ uint16(val)
		ctt.crcv = (ctt.crcv << 8) ^ ctt.ctbl[idx]
	}

	return len(blk), nil
}


func ( ctt * ccitt )Sum32() uint32  {
	var temp uint32

	temp = 0xCAFE
	temp = (temp << 16 ) | uint32( ctt.crcv )
	
	return temp;
}


func ( ctt * ccitt )Sum(b []byte) []byte  {
	s := ctt.Sum32()
	return append( b, byte(s>>24), byte(s>>16), byte(s>>8), byte(s) )
}


func ( ctt * ccitt )Reset()  {
	ctt.crcv = 0
	ctt.offs = 0
	return
}


func ( ctt * ccitt )Size() int  {
	return 4
}


func ( ctt * ccitt )BlockSize() int  {
	return 1
}


func NewCCITT() hash.Hash32  {
	var ctt ccitt

	ctt.ctbl = []uint16 {
		0x0000, 0x1021, 0x2042, 0x3063,  0x4084, 0x50A5, 0x60C6, 0x70E7,
		0x8108, 0x9129, 0xA14A, 0xB16B,  0xC18C, 0xD1AD, 0xE1CE, 0xF1EF,
		0x1231, 0x0210, 0x3273, 0x2252,  0x52B5, 0x4294, 0x72F7, 0x62D6,
		0x9339, 0x8318, 0xB37B, 0xA35A,  0xD3BD, 0xC39C, 0xF3FF, 0xE3DE,

		0x2462, 0x3443, 0x0420, 0x1401,  0x64E6, 0x74C7, 0x44A4, 0x5485,
		0xA56A, 0xB54B, 0x8528, 0x9509,  0xE5EE, 0xF5CF, 0xC5AC, 0xD58D,
		0x3653, 0x2672, 0x1611, 0x0630,  0x76D7, 0x66F6, 0x5695, 0x46B4,
		0xB75B, 0xA77A, 0x9719, 0x8738,  0xF7DF, 0xE7FE, 0xD79D, 0xC7BC,

		0x48C4, 0x58E5, 0x6886, 0x78A7,  0x0840, 0x1861, 0x2802, 0x3823,
		0xC9CC, 0xD9ED, 0xE98E, 0xF9AF,  0x8948, 0x9969, 0xA90A, 0xB92B,
		0x5AF5, 0x4AD4, 0x7AB7, 0x6A96,  0x1A71, 0x0A50, 0x3A33, 0x2A12,
		0xDBFD, 0xCBDC, 0xFBBF, 0xEB9E,  0x9B79, 0x8B58, 0xBB3B, 0xAB1A,

		0x6CA6, 0x7C87, 0x4CE4, 0x5CC5,  0x2C22, 0x3C03, 0x0C60, 0x1C41,
		0xEDAE, 0xFD8F, 0xCDEC, 0xDDCD, 0xAD2A, 0xBD0B, 0x8D68, 0x9D49,
		0x7E97, 0x6EB6, 0x5ED5, 0x4EF4, 0x3E13, 0x2E32, 0x1E51, 0x0E70,
		0xFF9F, 0xEFBE, 0xDFDD, 0xCFFC, 0xBF1B, 0xAF3A, 0x9F59, 0x8F78,

		0x9188, 0x81A9, 0xB1CA, 0xA1EB, 0xD10C, 0xC12D, 0xF14E, 0xE16F,
		0x1080, 0x00A1, 0x30C2, 0x20E3, 0x5004, 0x4025, 0x7046, 0x6067,
		0x83B9, 0x9398, 0xA3FB, 0xB3DA, 0xC33D, 0xD31C, 0xE37F, 0xF35E,
		0x02B1, 0x1290, 0x22F3, 0x32D2, 0x4235, 0x5214, 0x6277, 0x7256,

		0xB5EA, 0xA5CB, 0x95A8, 0x8589, 0xF56E, 0xE54F, 0xD52C, 0xC50D,
		0x34E2, 0x24C3, 0x14A0, 0x0481, 0x7466, 0x6447, 0x5424, 0x4405,
		0xA7DB, 0xB7FA, 0x8799, 0x97B8, 0xE75F, 0xF77E, 0xC71D, 0xD73C,
		0x26D3, 0x36F2, 0x0691, 0x16B0, 0x6657, 0x7676, 0x4615, 0x5634,

		0xD94C, 0xC96D, 0xF90E, 0xE92F, 0x99C8, 0x89E9, 0xB98A, 0xA9AB,
		0x5844, 0x4865, 0x7806, 0x6827, 0x18C0, 0x08E1, 0x3882, 0x28A3,
		0xCB7D, 0xDB5C, 0xEB3F, 0xFB1E, 0x8BF9, 0x9BD8, 0xABBB, 0xBB9A,
		0x4A75, 0x5A54, 0x6A37, 0x7A16, 0x0AF1, 0x1AD0, 0x2AB3, 0x3A92,

		0xFD2E, 0xED0F, 0xDD6C, 0xCD4D, 0xBDAA, 0xAD8B, 0x9DE8, 0x8DC9,
		0x7C26, 0x6C07, 0x5C64, 0x4C45, 0x3CA2, 0x2C83, 0x1CE0, 0x0CC1,
		0xEF1F, 0xFF3E, 0xCF5D, 0xDF7C, 0xAF9B, 0xBFBA, 0x8FD9, 0x9FF8,
		0x6E17, 0x7E36, 0x4E55, 0x5E74, 0x2E93, 0x3EB2, 0x0ED1, 0x1EF0,
	}

	/**/
	ctt.crcv = 0
	ctt.offs = 0
	return &ctt
}



